MPG Comparison Calculator
Enter two cars' mpg, your yearly miles and gas price to compare their yearly and monthly fuel costs, gallons saved, and how soon a higher price pays back.
MPG Comparison Calculator
Your two cars
Your driving
The EPA label assumes 15,000. U.S. licensed drivers averaged 13,735 miles in 2024 (FHWA). Not sure? Work it out from two odometer readings.
Car A's price, and Car B's unless you enter one below. U.S. regular averaged $4.28 a gallon in the week of Oct 19, 2026 (EIA).
How long you expect to own it. The EPA window sticker compares 5 years.
Car A
Use the combined mpg from the window sticker or fueleconomy.gov, or your own figure from fill-ups.
Car B
Same as Car A: combined mpg.
Leave blank if both cars use the same fuel. For premium or diesel, enter that price. Week of Oct 19, 2026: premium $5.31, diesel $6.28 (EIA).
Break-even (optional)
Optional. The extra you'd pay to buy the car that's cheaper to fuel. With the same fuel price for both, that's the one with the higher mpg.
—
| Figure | Car A | Car B |
|---|---|---|
| Fuel economy | — | — |
| Fuel price | — | — |
| Gallons per 100 miles | — | — |
| Gallons a year | — | — |
| Fuel cost a year | — | — |
| Fuel cost a month | — | — |
| Fuel cost over 5 years | — | — |
Fuel only, at today's price for every year. Insurance, maintenance, resale value and price changes aren't included. Your real mpg will differ from the label.
Road44
Driving Hours Tracker & Logger
Log each car's miles and fuel in Road44
Name the vehicle on each drive, then add its distance, fuel added and fuel cost. Drive history and exports keep every drive on record.
Free · No sign-up needed
Road44, the driving log app mentioned below, is our app. Fuel prices are EIA's US averages for the week of Oct 19, 2026, and the vehicle figures come from fueleconomy.gov, checked in October 2026. Prices change, so enter the ones you pay.
How the comparison works
The calculator works through five steps:
- Miles a year ÷ mpg = gallons a year.
- Gallons a year × price per gallon = fuel cost a year.
- Car A's yearly cost minus Car B's = your yearly fuel savings.
- Each yearly cost ÷ 12 = the monthly cost.
- Yearly figures × the years you keep the car = the totals.
With the starting figures, 15,000 miles a year at $4.28 a gallon, Car A at 35 mpg burns 429 gallons a year and Car B at 50 mpg burns 300. That's $1,834 against $1,284, so Car B saves $550 a year, $45.83 a month and $2,750 over 5 years. The 35 and 50 mpg are a sample pair, not averages: they're the combined ratings of the 2026 Toyota Corolla LE and the 2026 Corolla Hybrid LE on fueleconomy.gov.
Each car's yearly cost is rounded to the dollar first, and every other money figure is worked out from those two numbers, so what you see adds up: $1,834 minus $1,284 is $550, and $550 × 5 is $2,750. fueleconomy.gov's hybrid calculator rounds yearly costs the same way. The monthly savings is the difference between the two monthly costs, each rounded to the cent.
To check the math against a government source, enter 15,000 miles, $3.60, 20 mpg and 30 mpg. The calculator shows $900 a year and $4,500 over 5 years, the same as fueleconomy.gov's Save Money example.
The MPG illusion: why gallons per 100 miles tells you more
Miles per gallon hides how much fuel you actually save. The fueleconomy.gov label guide says mpg "can be potentially misleading when consumers compare fuel economy improvements, particularly when they use it in place of fuel costs." Its example: going from 10 to 15 mpg saves "about 33 gallons" over 1,000 miles, while going from 30 to 35 mpg saves "only about 5 gallons." It adds that "A fuel consumption metric allows for more accurate energy usage comparisons among vehicles."
Gallons per 100 miles is that consumption figure, and it's in the calculator's table for each car. Here's how it falls as mpg rises:
| Fuel economy | Gallons per 100 miles |
|---|---|
| 15 mpg | 6.67 |
| 20 mpg | 5.00 |
| 25 mpg | 4.00 |
| 30 mpg | 3.33 |
| 35 mpg | 2.86 |
| 40 mpg | 2.50 |
| 50 mpg | 2.00 |
At 15,000 miles a year and $4.28 a gallon, three 10-plus mpg steps compare like this:
| Switch | Gallons saved a year | Fuel saved a year |
|---|---|---|
| 20 to 30 mpg | 250 | $1,070 |
| 30 to 40 mpg | 125 | $535 |
| 35 to 50 mpg | 129 | $550 |
The same 10 mpg is worth twice as much at the low end. In a household with two cars driven the same miles, the arithmetic usually favors replacing the thirstier one: a 20 mpg car swapped for a 30 mpg one saves more fuel than a 30 mpg car swapped for a 40 mpg one. The effect gets its name from a 2008 Science paper by Larrick and Soll, "The MPG illusion". When you compare two cars, look at the gap in gallons per 100 miles, not the gap in mpg.
When a pricier, more efficient car pays for itself
The break-even is the extra purchase price divided by the yearly fuel savings. The fueleconomy.gov hybrid calculator defines its payback period as "The number of years required for the fuel savings to equal the price difference." It pairs the 2026 Corolla Hybrid LE, listed at $26,270, with the 2026 Corolla LE at $24,895. Enter the $1,375 difference with the starting figures and the hybrid pays back in 2.5 years, then comes out $1,375 ahead after 5 years.
A bigger gap can take longer than you'll own the car. At $550 a year, a $5,000 difference takes 9.1 years, so after 5 years you'd still be $2,250 short. The calculator says so when the years you enter run out first.
Fuel is only part of what a car costs. The hybrid tool notes that it "compares vehicles based on fuel cost and vehicle price only. Other factors, such as insurance, maintenance, or resale value, are not considered since they can vary widely." This page leaves the same things out, along with loan interest on the extra price and fuel prices that change from year to year. For the full cost of a mile, including insurance, maintenance and depreciation, use the cost per mile to drive calculator.
The fuel type can flip the result. At 15,000 miles, a 40 mpg diesel at $6.28 a gallon uses 125 fewer gallons a year than a 30 mpg car on $4.28 regular, yet costs $215 more to fuel. A 35 mpg car on premium at $5.31 costs $136 a year more than a 30 mpg car on regular. Enter the second fuel's price as Car B's, and the calculator names the cheaper car to fuel, not the one with the higher mpg.
Checking the numbers against your own driving
Label figures are for comparing cars. As fueleconomy.gov puts it, they "may not accurately predict the average MPG you will get." For a car you already own, measure its real mpg from two full tanks with the MPG calculator, and work out your yearly miles from odometer readings with the annual mileage calculator. If you drive for a gig app, the gig driver hourly pay after gas calculator shows what fuel does to your hourly pay.
If you want a per-car record over time, Road44 is the driving log we built:
- Type the car's name in the Vehicle field on each drive. The five most recent names come back as one-tap suggestions.
- Add the distance from start and end odometer readings or as a typed number. On the drive you filled up on, add Fuel added and Fuel cost.
- Drive history, the calendar and the Text, CSV, PDF or PNG export are free. Turn on the Vehicle chip in Export to add each drive's car.
- The Insights screen (PRO) works out an overall fuel economy from drives that have both fuel added and a distance, and its By vehicle card splits your hours, distance, earnings or trips by car.
There's no GPS: every drive is started, stopped or typed in by hand. Road44 is a personal record, not an official or certified logbook, and drives stay on your phone with no account.
Frequently Asked Questions
Common questions about mpg comparison calculator
How do I compare the fuel cost of two cars?
Divide your yearly miles by each car's mpg to get the gallons it burns, then multiply by the price per gallon. The difference between the two is your yearly fuel savings, and dividing by 12 gives the monthly figure. In fueleconomy.gov's own example, a car that gets 30 mpg "will cost you $900 less to fuel each year than one that gets 20 MPG" at 15,000 miles a year and $3.60 a gallon, and "Over a period of 5 years, the 30-MPG vehicle will save you $4,500." At $4.28 a gallon, the US average for regular in the week of Oct 19, 2026, the same pair saves $1,070 a year and $5,350 over 5 years.
Why does going from 20 to 30 mpg save more than going from 30 to 40?
Because the gallons you burn fall on a curve as mpg rises. At 15,000 miles a year, going from 20 to 30 mpg cuts fuel from 750 to 500 gallons, a saving of 250. Going from 30 to 40 mpg cuts it from 500 to 375, a saving of only 125. At $4.28 a gallon that's $1,070 a year against $535. fueleconomy.gov's label guide calls this the "MPG illusion": 10 to 15 mpg saves "about 33 gallons" over 1,000 miles, while 30 to 35 mpg saves "only about 5 gallons." It says it "may be more meaningful to express fuel efficiency in terms of consumption (e.g., gallons per mile or per 100 miles) rather than in terms of economy (miles per gallon)." The term comes from a 2008 Science paper by Larrick and Soll, "The MPG illusion". That's why the calculator shows gallons per 100 miles: 20 mpg is 5.00, 30 is 3.33 and 40 is 2.50, so the first step saves 1.67 gallons every 100 miles and the second only 0.83.
How long until a more efficient car pays for itself?
Divide the extra purchase price by the yearly fuel savings. Take the 2026 Toyota Corolla Hybrid LE ($26,270 MSRP, 50 mpg combined) and the 2026 Corolla LE ($24,895, 35 mpg), a $1,375 gap as listed in October 2026. At 15,000 miles and $4.28 a gallon, the hybrid costs $550 a year less to fuel, so it pays back in 1,375 ÷ 550 = 2.5 years and is $1,375 ahead after 5 years. MSRPs and gas prices change, so use the figures in front of you. The fueleconomy.gov hybrid calculator, which pairs those two cars, defines the payback period as "The number of years required for the fuel savings to equal the price difference." It also says it "compares vehicles based on fuel cost and vehicle price only. Other factors, such as insurance, maintenance, or resale value, are not considered since they can vary widely." If you borrow the extra, the loan interest adds to the price difference.
What if one car takes premium or diesel?
Enter that car's price as Car B's fuel price. EIA's US averages for the week of Oct 19, 2026 were $4.284 a gallon for regular and $6.276 for diesel (Gasoline and Diesel Fuel Update), and $5.314 for premium (prices by grade). A pricier fuel can wipe out an mpg lead. At 15,000 miles, with regular at $4.28 and diesel at $6.28, a 40 mpg diesel uses 125 fewer gallons a year than a 30 mpg gas car but costs $215 more to fuel, because diesel cost 46% more per gallon that week. With premium at $5.31, a 35 mpg car on premium costs $136 a year more than a 30 mpg car on regular. If Car A is the one on premium, put the premium price in the main price field and the regular price in Car B's.
Which mpg should I enter for each car?
For a car you're shopping for, use the combined mpg from the window sticker or fueleconomy.gov's Find a Car. As the label guide puts it, "Combined fuel economy is a weighted average of City and Highway MPG values that is calculated by weighting the City value by 55% and the Highway value by 45%." For a car you already drive, your own figure from fill-ups is better. Our MPG calculator works it out, and it converts UK mpg and L/100 km to US mpg too. fueleconomy.gov says EPA ratings are "a useful tool for comparing the fuel economies of different vehicles but may not accurately predict the average MPG you will get." Use the same kind of figure for both cars, both from the label or both measured, or the comparison tilts toward one of them.
How many miles a year should I use?
Your own, if you can: two odometer readings a known time apart give it, and the annual mileage calculator does the math. The EPA label assumes 15,000 miles a year, and FHWA reports that licensed drivers averaged 13,735 miles in 2024. Savings grow in step with miles: the default pair saves $550 a year at 15,000 miles and $504 at 13,735. A household's second car may do far fewer miles, which stretches any break-even.
Why doesn't this match the "You save" figure on the window sticker?
The sticker compares one car to the average new vehicle, not to the car you're weighing it against. According to fueleconomy.gov, it reads "You save $x,xxx in fuel costs over 5 years compared to the average new vehicle," and "These estimates are based on driving 15,000 miles per year, for five years, and the projected fuel price for the year ($3.70 per gallon of gasoline in this example)." This calculator compares your two cars at the miles and fuel price you type, so its figures follow the price you enter, not the sticker's projected one.
Can I track each car's real fuel costs in Road44?
Yes, by hand. Type the car's name in the Vehicle field on each drive; the five most recent names come back as one-tap suggestions. Add the distance from start and end odometer readings or as a typed number, and on the drive you filled up on, add Fuel added and Fuel cost. Units are miles or kilometers and gallons or liters. Drive history, the calendar and the Text, CSV, PDF or PNG export keep the record, and turning on the Vehicle chip in Export adds each drive's car. Road44's Insights screen (PRO) works out an overall fuel economy from drives that have both fuel added and a distance, and its By vehicle card splits your hours, distance, earnings or trips by car. There's no GPS: every drive is started, stopped or typed in by you, and the log is a personal record, not an official one.